It is often desirable to bore through a hard earth formation with a drill bit having roller cone cutters designed to scrape and gouge the formation. A cone cutter having broad, flat milled teeth can very effectively scrape and gouge such formations. However, because milled teeth are formed integrally with the surface of the cone cutter, they are typically formed from materials that wear quickly while boring through hard formations. Even when coated with an abrasion-resistant material, milled teeth often crack or break when they encounter hard formations. Thus, milled teeth are typically unsuitable for boring through hard earth formations.
To replace milled teeth in hard-formation cone cutters, engineers developed cylindrical cutter inserts that are formed from a hard, abrasion-resistant material such as sintered and compacted tungsten carbide. Typically, such inserts or compacts have a generally frustoconical or chisel-shaped cutting portion and a cylindrical base. The base is fitted into a socket, which is drilled into the exterior of the roller cone cutter, such that the cutting portion protrudes from the exterior of the associated cone cutter.
Cone cutters having hard-earth cylindrical inserts with frustoconical cutting portions tend to crush the formation instead of scraping and gouging it. Thus, although less prone to wear and breakage than milled teeth, hard-earth inserts having frustoconical cutting portions do not provide the desired cutting action.
Cone cutters having hard-earth cylindrical inserts with chisel-shaped cutting portions often cannot scrape and gouge a hard earth formation as effectively as cone cutters having milled teeth. Within a row of cutter inserts, the sockets are separated by a minimum distance or clearance in order that expected drilling forces do not deform the sockets. Such deformation might allow the insert to rotate within or become dislodged from its respective socket. Because of this minimum distance and because the length of the chisel crest is limited by the diameter of the insert's cylindrical base, cylindrical inserts often cannot be made with chisel crests long enough to provide a scraping and gouging action that is as effective as that provided by milled teeth.
Because of the cylindrical shape of the base and socket, a cylindrical insert may tend to rotate within its socket. This rotation may orient a chisel-shaped cutting portion so as to further reduce the gouging and scraping effectiveness and the penetration rate of the cone cutter. Furthermore, such rotation over an extended period may dislodge the insert from the socket. Following are examples of prior cutter inserts.
U.S. Pat. No. 3,599,737 to John F. Fischer, patented Aug. 17, 1971, discloses a hardened metal insert with out-of-round abutment portions. The inserts are press-fitted into respective sockets formed in the associated cone cutter. Then, the cone cutter surface adjacent the abutment portions is staked to displace metal into the abutment portions to prevent axial and rotational displacement of the insert. Providing the abutment portions and the staking represent additional manufacturing steps. Furthermore, the frustoconical cutting portion provides a crushing action instead of a scraping and gouging action.
U.S. Pat. No. 3,749,190 to Clarence S. Shipman, patented Jul. 31, 1973, discloses a tapered carbide button insert. The button insert is fitted into a socket formed in a rock drill bit. A sleeve is then forced into the gap between the insert and the socket wall and extruded into an undercut of the socket. By virtue of its shear strength, the sleeve retains the insert in the socket.
However, the installation of the sleeve represents an additional manufacturing step, and the button insert fails to provide a scraping and gouging action.
U.S. Pat. Nos. 4,406,337 to Herbert C. Dill, patented Sep. 27, 1983, discloses a cutter insert having at least two projections protruding from the bottom of its base. When the insert is fully pressed into a respective socket, the projection becomes embedded in the socket bottom to prevent rotation of the insert.
However, if the insert dislodges enough to disengage the projections from the socket bottom, the projections can no longer prevent rotation of the insert. Furthermore, the chisel crest is constrained to a shorter-than-desired length by the diameter of the insert base.
U.S. Pat. No. 3,389,761 to Eugene G. Ott, patented Aug. 26, 1968, discloses a carbide insert having alternate ridges and valleys sized to engage the socket walls. The interference between the ridges and the socket walls helps to prevent the insert from rotating within the socket, and thus helps to retain the insert within the socket.
However, with continued use, the interference between the ridges and the socket walls may weaken to a level insufficient to prevent rotation of the insert. Again, the crest length is limited by the diameter of the cylindrical insert base.
Other inserts are disclosed in U.S. Pat. Nos. 4,047,583; 4,420,050; 4,271,917; 4,254,840; 4,176,725; and 4,086,973. These inserts have shortcomings similar to those described above.
None of the above-mentioned references have provided a way of increasing the crest length of the insert's chisel portion without increasing the diameter of the insert's base, and hence, without decreasing the maximum possible number of inserts within an annular row. Furthermore, none of the above-mentioned references have provided means for efficient manufacturing of a cone cutter with inserts while preventing rotation of such inserts within their respective sockets.